Alter Rust output format to be standalone

[str4d]

Currently, the Rust output format is intended to be copy-pasted into a #[test] function in other codebases. That was fine for small test vectors, but we have much larger ones now, and so it makes more sense to have the output be a valid standalone Rust file.

We should use the following format style:

pub(crate) struct TestVector {
    pub(crate) NAME: TYPE,
    ...,
}

// From https://github.com/zcash-hackworks/zcash-test-vectors/...
pub(crate) const TEST_VECTORS: &[TestVector] = &[
    TestVector {
        NAME: VALUE,
        ...,
    },
];